7

THE PERCEPTION OF MOTION

The moon seems to be sailing 
through the clouds. Most of us 
have experienced this illusion, 
but few ask why it occurs. Of 
course, the moon does change 
its position in the sky as the 
earth rotates, but that 
movement is too slow to detect 
with the eye. In fact, we 
perceive the moon to move in a 
direction opposite to that of the 
clouds passing in front of it, 
regardless of the direction in 
which the clouds happen to be 
traveling.

    Ever since the discovery in 
the nineteenth century of 
illusory stroboscopic effects, 
the predecessors of moving 
pictures, motion perception 
has been one of the major areas 
of investigation in vision. Like 
form or color, motion is a 
perceptual property of objects. 
Although the second hand and 
the minute hand of a watch 
both move, the second hand is 
seen to be moving whereas the 
minute hand moves at a rate 
usually below our threshold for 
detecting its motion. Although 
it is the clouds that often move 
at a rate that we can detect, it 
is the moon that we perceive as 
moving. The perception of 
motion, then, is not simply a 
reflection of the physics of 
motion, of what is happening 
in the world. Although in 
physics one might say that no 
object moves absolutely but 
only changes its position 
relative to some frame of 
reference, in perception 
objects do appear to move 
absolutely or to be stationary.